The present invention relates generally to an elastomeric material composition for use in joining heat-dissipating devices with heat generating electronic devices and a method for manufacturing the same. More particularly, this invention relates to a new compressible thermal interface assembly having an integral interface and fastening means that is applied directly to the heat dissipation device at the time of manufacture. The present invention includes a interface composition that contains thermally conductive filler material in a conformable elastomeric matrix and an integral means for adhering the heat dissipation device to a heat-generating surface thereby compressing the interface composition to form an improved heat sink device with an integral, compressible thermally conductive interface layer. Further, a method of manufacturing the device is also provided.
In the prior art, it is well known that the most critical locations that effect the overall performance of a heat transfer assembly are the interface points. These locations are where two different materials mate to one another introducing two contact surfaces and often an air gap across which the heat being dissipated must be transferred. Generally, the contact surfaces are not always perfectly flat due to milling or manufacturing tolerances thus creating small and irregular gaps between the heat generating surface and the heat dissipating devices thereby increasing the thermal resistance of the overall assembly. These imperfections and gaps between the mating surfaces often contain small pockets of air that can significantly reduce the heat transfer potential across the interface between the heat generating surface and the heat-dissipating device.
Various materials have been employed in the prior art in an attempt to bridge this interface gap. In particular, organic base materials such as polysiloxane oils or polysiloxane elastomeric rubbers and thermoplastic materials such as PVC, polypropylene, etc. loaded with thermally conducting ceramics or other fillers such as aluminum nitride, boron nitride or zinc oxide have been used to impart thermally conducting properties to the organic base material. In the case of polysiloxane oils loaded with thermally conducting materials, these materials are applied by smearing the heat sink or other electronic component with the thermally conducting paste and then securing the heat sink in place by mechanical means using clips or screws. These prior art, thermal greases show superior film forming and gap filling characteristics between uneven surfaces thus providing an intimate contact between the surface of the heat sink and the surface of the heat-generating source. However, it has been found that the thermal greases exhibit poor adhesion to the surfaces of the heat sink and heat generating surface, thus effectively seeping out from between the heat sink and the heat-generating surface, causing air voids to form between the two surfaces that eventually result in operational hot spots. Moreover, excessive pressure placed upon the heat sink by the mechanical fasteners accelerates this seepage from between the heat sink and the surface of the heat-generating surface. It has been reported that excessive squeeze out of polysiloxane oils can evaporate and recondense on other sensitive parts of the surrounding microcircuits. The recondensed oils lead to the formation of silicates that potentially interfere with the function of the microprocessor, eventually causing failure of the system.
In the case of polysiloxane rubbers and thermoplastic polymers, these materials are typically cast in sheet form and die cut into shapes corresponding to the shape of the heat sink and heat generating device. The resulting preformed sheet is then applied to the surface of the heat-generating surface securing the heat sink by means of clips or screws. The precut films solve the problems associated with greases but do not provide adequate intimate contact required for optimum heat transference between the heat generating source and the heat sink. The added step of cutting preforms and manually applying the pad adds cost to the assembly process. Furthermore, these types of materials show variable performance due to variation in the thickness of the pad and the amount of pressure applied to the thermally conducting precut film, based upon the mechanical device or action used to secure the heat sink. Further, while these known interface materials, are suitable for filling undesirable air gaps, they are generally are less thermally conductive than the heat sink member thus detracting from the overall thermal conductivity of the assembly.
An additional drawback to most of the above noted interface materials is that they require a machined heat sink be secured to a heat generating surface or device using mechanical clips or screws adding to the complexity and assembly time for the overall assembly.
In an attempt to overcome the requirement of mechanical fastening some prior art thermal interface pads are formed of a material that is soft and pliable, having an adhesive on both sides. The pad is first applied under pressure to the mating surface of the heat-dissipating device and the assembly is then pressed onto the heat-generating surface. The pliability of the interface material allows the pad to be compressed into the small grooves and imperfections on the two mating surfaces thus improving the overall performance of the heat transfer through the interface area. The drawback in the prior art is that the use of an adhesive interface pad requires an additional fabrication/assembly step and introduces an additional layer of material along the heat dissipation pathway. Further, as mentioned above, since all of the materials within the assembly are different, optimum heat transfer cannot be achieved.
Therefore, in view of the foregoing, heat transfer assemblies that include interface pads that are formed integrally with the interface contact surface that include a means for mounting the assembly in compression with a heat-generating surface are highly desired. There is also a demand for a heat dissipating assembly for use in an electronic device that is lightweight, has an integral compressible interface pad material and fastening means that can be applied directly to complex geometries for accurate mating of the interface surfaces.